A calibration system that calibrates robots installed in a process includes: a reference measuring element; a flange measuring element attached to a flange provided at an arm tip of each robot; a fixed measuring device configured to measure the flange measuring element of each robot and the reference measuring element; and a controller configured to control the robots, calculate a position and attitude relation between the flange measuring element of each robot and the reference measuring element based on a measured result measured by the fixed measuring device, and calculate an error between a design installation position and attitude of each robot and an actual installation position and attitude of the robot by using the measured position and attitude relation and position and attitude data of the flange measuring element in a robot coordinate system at the time when the robot has been measured by the fixed measuring device.